The present invention relates generally to discharge lamps. More particularly, the invention relates to an improved electron emissive material for the electrodes of such lamps.
The present invention will be described primarily in the context of a sodium vapor discharge lamp. However, the intended scope of the invention is not limited to such lamps but also covers other vapor discharge lamps, including without limitation, HCRI sodium, unsaturated vapor sodium, fluorescent, and high pressure mercury. Moreover, the present emission mix is suited to any discharge lamp in which an alkali metal is present in the vapor phase.
One of the primary requirements in a vapor discharge lamp is the establishment of the arc. To facilitate this step in the process of creating light, the electron emitting electrodes used in most discharge lamps are treated with a coating of an emission material, generally a refractory powder mixture, which lowers the work function of the electrode so that a lower voltage may be used to start the arc. The process of lowering the starting voltage by applying lower work-function materials on the electrode surface is known as "activation" and an extensive understanding of cathode activation theory and practice has developed. In fact, many electrodes used in devices unrelated to vapor discharge lamps include materials to lower the work function, see U.S. Pat. No. 3,849,690.
An early understanding of "activation" resulted from use of certain materials with the cathode of a vacuum tube. For example, U.S. Pat. No. 4,076,992 discloses the use of a barium oxide, aluminum oxide, and calcium oxide emission mix. However, this disclosure does not contemplate the generation of light. Discussion of the generation of light through electron initiated discharge and the associated use of an emission mixture is found in U.S. Pat. Nos. 3,026,177; 3,026,210; 3,453,477; 3,485,343; 3,708,710; 3,935,494; 4,052,641; 4,079,167; 4,150,317; 4,155,758; 4,251,569; 4,285,372; 4,313,854; 4,319,158; 4,374,339; 4,468,586 and 4,620,128. Each of these patents is herein incorporated by reference.
One the major factors limiting the life of lamps employing a sodium discharge is the loss of sodium from the discharge. When the loss of sodium from the vapor phase in the lamp is too large, the color and light output become unsuitable. Thus, although the lamp operates well initially, the useful life of the lamp may be so limited as to be impractical.
In addition, electrode emission materials used in the standard HPS lamps have relatively high sodium consumption rates but it does not affect the claimed lamp life, 24000 hours, due to the relatively high sodium content. The disadvantage of these lamps is that they also have a high Hg content which gives rise to the familiar and annoying cycling behavior at the end of the lamp life. To avoid cycling, the mercury and sodium doses must be reduced by factor of 5 to 10 and all the sodium loss mechanisms must be reduced in order to achieve longer life.
Early electrode emissive material, in fact, one now in general use in the lighting industry for standard HPS lamps, is comprised of barium calcium tungstate as described in U.S. Pat. No. 4,617,492. Nonetheless, the standard tungstate emissive materials are too reactive with sodium to provide long life and non-cycling HPS lamps.
Along these lines, U.S. Pat. No. 5,111,108 describes an emission mix intended to provide very low sodium reactivity. More particularly, this patent describes an emissive material comprised of a reacted mixture of barium-strontium-yttrium oxides. Although this composition is believed to provide certain benefits, it also has been recognized that commercial synthesis of phase pure compounds of these oxides can be difficult, and the yttrium component is relatively rare and therefore expensive.
In copending U.S. Ser. No. 08/796,669 it is disclosed that Ba.sub.3 Al.sub.2 O.sub.6 provides a superior electron emission mix for a sodium vapor discharge lamp. Nonetheless, it remains desirable in this art to have an effective alternate emission mix which lowers the required start-up voltage yet is resistant to reaction with sodium in the vapor discharge. The present invention, utilizing a different oxide of barium and aluminum has unexpectedly demonstrated superior results with respect to reactivity with sodium.